She nearly cheated

My wife and I were married very young. She was 18 and I was 22. It was a bit of a shotgun wedding as she fell pregnant (Unplanned) We have just celebrated 11 years together in August. We have shared many good times and also some trying times in our relationship but never have I felt insecure and always felt we were great together.

Just before Christmas last year, my wife went out on the town with some girls from work. As far as I was aware, the night was uneventful. On the 6th of January, my wife was playing netball and I was home with our kids. For some reason I looked on her Facebook account at her messages and to my surprise I found a rather suspicious conversation between her and some guy. She had obviously been deleting the conversation on a regular basis as all I could see was the following -
Her - "You didn't have to go all quiet on me"
Him - "Haha what are you wearing?"
Her - Sorry last night wasn't a good night..... I hope you survived without me.

When she came home I asked her who the guy was she had been chatting to on Facebook. She looked a little panicked and said "Oh that's Drew, I went to school with him and we have just been chatting" She also said "I seen him when I was out the other week and I tried to hook him up with my single girlfriend"

I then sat her down at the computer and asked her to explain each sentence in the message. I also told her a bit of a lie and said I had the ability to un-delete all her conversation history if I wanted to....... She started crying and admitted they had been flirting over Facebook since she had seen him at the night club 2 weeks earlier. She said he had hit on her and become quite sleazy at the club and had then messaged her to apologize the next day. Since then they started conversing and it had become very flirtatious. She admitted that one night when I was at work, he was asking to come over and hook up. She never told him "No" she just didn't reply and told him the next day that she had fallen asleep.

She was very upset that she had been discovered and assured me that nothing physical had ever happened between them. I forgave her and after several days, alot of crying and talking about it we moved on. She said the only way she could explain her action was that it made her feel like a school girl getting attention. I asked her what would of happened if I never discovered what was happening and she said that if she was honest she couldn't say for sure if it would of eventually lead to something physical. In the 7 months since, we have been more loving and affectionate to each other than ever before. Our relationship feels very real and intimate and we are like best friends. I promised her that I would not bring up the past during an argument as I had forgiven her. The problem is, just when I think I've moved on I go thru periods of anger, insecurity and depression over the whole thing.

I constantly have questions running thru my head like -
Why did she do it?
Do I know the whole truth?
Did anything more happen between them?
What would of happened if not for me finding out?

The thought of them messaging for 2 weeks makes me feel sick......... I want to read what was written but it cannot be undeleted on FB.

How do I move on without dredging up the past?